1. Field of the Invention
The present invention relates to a robot and a robot control method that use string-shaped members, such as wires and belts, as transfer mechanisms of driving force.
2. Description of the Related Art
In recent years, realization of a robot capable of joint work with a human is desired. A manipulator used in the robot not only needs to be capable of highly accurate positioning as in a conventional manipulator, but also needs to be flexible upon collision with a human.
A method of using an actuator including a variable stiffness mechanism to control stiffness of a joint is known as a method of realizing the flexible manipulator. Also known is a method of using wires or belts to transfer driving force to a joint to thereby set an actuator on a base to reduce mass of a movable section. Therefore, both a variable stiffness actuator and a wire driving mechanism can be used to realize a manipulator with significantly high flexibility.
Conventionally, proposed is a technique for realizing a variable stiffness mechanism by inserting a non-linear spring element to a path for transferring driving force using a wire, wherein the elastic coefficient of the non-linear spring element changes in proportion to tensional force (see Park et al., “Optimization of Tendon-Driven Robot Joint Stiffness using GA-based Learning”, Journal of Robotics Society of Japan, May 15, 2006, Vol. 24, No. 4, pp. 482 to 488). A wire tensional force reference value for simultaneously realizing trajectory tracking control of hand and control of joint stiffness is computed through learning using a genetic algorithm.
Also proposed is a robot with a wire used as a variable stiffness mechanism, wherein the elastic coefficient of the wire non-linearly changes in proportion to the amount of expansion and contraction (see Japanese Patent Application Laid-Open No. 2006-35325). A feedback controller is used to cause a joint angle to follow a target trajectory. However, in feedback control, if the natural frequency of one of vibration perpendicular to the expansion/contraction direction of the wire (hereinafter, called “string vibration”) and vibration around the joint caused by elasticity of the wire is equal to or smaller than a control bandwidth, the controller excites the vibration of the wire, and the control system becomes unstable. Therefore, in Japanese Patent Application Laid-Open No. 2006-35325, the control bandwidth of the feedback controller is limited to stabilize the control system if the natural frequency of the wire decreases as a result of a reduction in the tensional force of the wire. Since the stiffness is not controlled in Japanese Patent Application Laid-Open No. 2006-35325, the joint stiffness is determined by control input for position feedback.
However, the tensional force of the wire for realizing desired joint stiffness is computed through learning in Park et al., “Optimization of Tendon-Driven Robot Joint Stiffness using GA-based Learning”, Journal of Robotics Society of Japan, May 15, 2006, Vol. 24, No. 4, pp. 482 to 488. Therefore, repetitive calculation needs to be performed again to change the target value of the joint stiffness, and the target value cannot be easily changed. Furthermore, since only the feedforward controller is used to perform tracking control of the target trajectory, the positioning accuracy is significantly reduced due to a model error or a disturbance to a link.
Meanwhile, in Japanese Patent Application Laid-Open No. 2006-35325, the positioning accuracy is degraded if the control bandwidth is reduced as a result of the reduction in the tensional force of the wire. Therefore, the realization of highly accurate positioning control is difficult in the techniques. Furthermore, since the joint stiffness is not controlled in Japanese Patent Application Laid-Open No. 2006-35325, the elastic coefficient of the wire increases if a large tensional force command value is output in the positioning control, and the flexibility of the joint is lost.
An object of the present invention is to provide a robot and a robot control method capable of highly accurate positioning of a link without losing flexibility of a joint.